1. Field of the Invention
The present invention relates to an electroacoustic transducer, and, more particularly, to an electroacoustic transducer, which has an outer case formed integral with lead terminals and a coil having both coil terminals connected to lands of the lead terminals wherein the shapes of the lands of the lead terminals are specifically designed to improve the quality of the connecting portions.
2. Description of the Related Art
An electroacoustic transducer, such as an electromagnetic type electroacoustic transducer, has a structure as shown in FIGS. 19 through 23. As shown in FIGS. 20 and 21, a case 201 has a base member 203 attached to its bottom. A base 205 and a core 207 are integrally secured on the base member 203 and inside the case 201. The integration of the base 205 and the core 207 is called "pole piece." A coil 209 is wound around the core 207. A support ring 211 is disposed around the coil 209, and a magnet 213 is provided on the inner wall of the support ring 211. A ring-like clearance is formed between the magnet 213 and the coil 209. Formed at the left-hand end portion of the support ring 211 in FIG. 21 is a step portion 215 at which a diaphragm 216 is provided. This diaphragm 216 comprises an elastic plate (also called a resonance plate) 217 and a magnetic piece 219 attached as an added mass to the center portion of this elastic plate 217.
In the thus constituted electromagnetic type electroacoustic transducer, as shown in FIG. 20, lead terminals 221 and 223 have previously been attached in an integral manner to the base member 203 by inserting. In this case, as shown in FIG. 20, both coil terminals 209a and 209b of the coil 209 are led out on lands 221a and 223a of the lead terminals 221 and 223 and are securely soldered to those lands.
The aforementioned inserting method will now be discussed specifically. As shown in FIG. 19, a lead frame 225 previously patterned in a predetermined shape is prepared. The lead frame 225 has a plurality of lead frame elements 227 (surrounded by an alternate one long and two short dashes line in FIG. 19) coupled side by side, so that a plurality of electromagnetic type electroacoustic transducers (four electroacoustic transducers in this case) are manufactured at the same time. FIGS. 22 and 23 show the details of the lead frame elements 227. The lead frame element 227 has a pair of wide portions 227a and 227b, extending horizontally in FIG. 22, and protruding pieces 227c, 227d, 227e and 227f are provided between the pair of wide portions 227a and 227b. The distal end portions of the protruding pieces 227c and 227d become the aforementioned lead terminals 221 and 223.
As shown in FIG. 19, the constituting elements of each electromagnetic type electroacoustic transducer are placed at the proper positions of the lead frame elements 227 of the lead frame 225 in a mold (not shown). Then, a resin is filled in the mold to form the aforementioned base member 203, at which time the lead frame elements 227 partially become integrated with the interior of the base member 203. Thereafter, the lead frame elements 227 are cut along a cut line A shown in FIG. 19 and the exposed portions are bent, providing the state shown in FIG. 20.
Besides the lead terminals 221 and 223, terminal members 229 and 231 which do not perform electric functions are likewise integrated with the base member 203. That is, the distal end portions of the protruding pieces 227e and 227f shown in FIG. 22 become the terminal members 229 and 231.
In the thus constituted electromagnetic type electroacoustic transducer, the elastic plate 217 integrally provided with the magnetic piece 219 is set to have a given polarity by the magnet 213. When a current flows across the coil 209 via the lead terminals 221 and 223 under this situation, the core 207 is magnetized, generating a magnetic field at the distal end. When the magnetic pole of the core 207 induced by the coil 209 is different from the magnetic pole induced by the magnet 213 attached to the elastic plate 217, the elastic plate 217 is attracted to the core 207. When the former magnetic pole of the core 207 is the same as the latter magnetic pole induced by the magnet 213, the elastic plate 217 is repelled against the core 207. By allowing the current to intermittently flow in either direction, therefore, the elastic plate 217 repeats the above-described operation. In other words, the elastic plate 217 vibrates at a given frequency, thus generating a sound.
This electromagnetic type electroacoustic transducer is then to be incorporated into any desired device, such as a portable telephone or a pager. To accomplish this, as shown in FIG. 21, this electroacoustic transducer is attached to a mounting board 237 (indicated by an alternate one long and two short dashes line in FIG. 21) of the device in the illustrated manner, and is soldered thereto via the lead terminals 221 and 223 and the lead terminals 229 and 231 which do not perform electric functions.
This kind of electromagnetic type electroacoustic transducer is disclosed in, for example, Unexamined Japanese Patent Publication No. Hei 5-80774.
The above-described prior art structure has the following shortcoming. As has already been described, both coil terminals 209a and 209b of the coil 209 are led out on the lands 221a and 223a of the lead terminals 221 and 223 and securely soldered there. The coil 209 has an insulator coat of polyurethane, polyester or the like applied on the outer surface of a copper wire with an outside diameter of 0.05 to 0.06 mm, and further has a deposited coat of modified nylon or the like applied on the insulator coat. The peripheral portions of the lead terminals 221 and 223 are made of a material susceptible to heat, such as thermoplastic resin. In respectively soldering the coil terminals 209a and 209b to the lands 221a and 223a, first, the insulator coats and deposited coats of the coil terminals 209a and 209b are melted away, thus exposing the copper wires. Under this condition, the coil terminals 209a and 209b are soldered to the respective lands 221a and 223a. This soldering should be performed quickly because the peripheral portions of the lands 221a and 223a are made of thermoplastic resin which is susceptible to heat.
In the case where soldering is carried out on the lands 221a and 223a, a solder paste when dropped on the lands 221a and 223a becomes as illustrated in FIG. 24. Solder naturally spreads in a circular shape or a dome shape. As the conventional lands 221a and 223a have approximately rectangular shapes, the surface tension strongly acts in the lengthwise direction of the lands 221a and 223a so that the solder spreads in this direction, as shown in FIGS. 24 and 25. The insulator coat and deposited coat portions of the coil terminals 209a and 209b are not yet melted away in the initial stage of soldering, and the solder is not yet wetted at those insulator coat and deposited coat portions (the solder is not yet well applied). As a result, "solder splitting" (which causes solder to be split into two in the lengthwise direction with the insulator coat and deposited coat portions as the boundary) as shown in FIGS. 25 and 26. When such solder splitting occurs, the coil terminals 209a and 209b are not properly soldered to the respective lands 221a and 223a. This improper connection leads to a loss of the electric function.
One reason why the lands 221a and 223a are designed to be approximately rectangular is its easier positioning of the coil terminals 209a and 209b on the respective lands 221a and 223a. In other words, even when the positions of the coil terminals 209a and 209b led on the respective lands 221a and 223a are slightly deviated from the center positions, the effective soldering area of each land does not vary so much.
To suppress the occurrence of the aforementioned solder splitting, the insulator coat and deposited coat portions of the coil terminals 209a and 209b should surely be melted away, which requires a longer heating time in the soldering job. The increased heating time may thermally damage the peripheral portions of the lands 221a and 223a made of thermoplastic resin. In this respect, the heating time cannot be made longer after all.
Alternatively, the amount of solder used may be increased in which case the solder height at the soldered portion becomes higher, reducing the volume of the resonance space in the case 201. This results in an insufficient resonance effect. Further, the increased solder height at the soldered portion stands in the way of making an electromagnetic type electroacoustic transducer thinner and more compact.
The occurrence of solder splitting may be prevented by repeating quick heating at the soldering time. In this case, the soldering work becomes troublesome and takes time.
As apparent from the above, it is significantly difficult to suppress the occurrence of solder splitting while maintaining the properness of the peripheral portions of the lands 221a and 223a, which are susceptible to heat, ensuring a thinner and more compact electromagnetic type electroacoustic transducer without degrading its performance, and avoiding making the soldering work troublesome.